This project is devoted to the basic biology of intracellular parasite-host cell interaction. Representative obligately intracellular procaryotes occupying different intracellular compartments within the eucaryotic cell are being examined to allow comparison of the varied mechanisms employed by these microorganisms to allow survival and growth within eucaryotic cells. In the past year, the bioenergetic properties of Coxiella burnetii, the etiological agent of Q fever, have been described. These studies demonstrate a physiological basis for the extreme resistance of C. burnetii to environmental conditions while providing for an activation of metabolism upon ingestion into the acidic and normally bactericidal phagolysosome. Currently, initial events involved in the interaction of parasites with the host are under investigation. The following observations have been made: (i) the major outer membrane protein (MOMP) of chlamydia is extensively cross-linked via disulfide bands, (ii) the extent of this cross-linking varies between the infectious elementary body and the differentiated reticulate body, and (iii) cleavage of surface-exposed portions of this protein by a variety of proteases did not reduce infectivity or rates of association with tissue culture cells. From these studies, it was concluded that structural stability of chlamydia, which had peptidoglycan, was conferred by cross-linking of MOMP, but that surface-exposed protease sensitive portions of this protein are not required for attachment, entry, blockage of phagosome-lysosome fusion or differentiation to the replicating reticulate body.